1. Field of the Invention
The present invention relates generally to explosive detonators and, more particularly, is concerned with a low-voltage detonator providing improved electrostatic discharge, or spark, safety.
2. Description of the Prior Art
Reliable low-voltage detonators are typically loaded either with primary explosives, commonly lead azide and/or lead styphnate, or more recently with CP (2-(5-cyanotetrazolato) pentaamminecobalt (III) perchlorate) because it provides some safety advantages over the previously-used primary explosives. However, detonators containing CP or primary explosives adjacent to an electrical ignition device, such as bridgewire, lack intrinsic spark safety. A human-body-equivalent electrostatic discharge between a pin and the electrically-isolated housing of the detonator is sufficient to ignite the energetic material and yield a detonation output.
As a result, such detonators lack intrinsic electrostatic discharge protection and so external design features such as spark gaps, varistors, or electrostatic shunt mixes must be incorporated. In addition, CP and primary explosives readily autoignite. Consequently, detonators that contain these materials commonly yield detonation output when heated rapidly, as in a bonfire scenario.
Various attempts have been made to develop a spark-safe, low-voltage detonator by loading the detonator with an organic, secondary explosive, such as PETN (pentaerythritol tetranitrate), HMX (cyclotetramethylenetetranitramine), or RDX (cyclotrimethylene trinitramine). Such materials should provide intrinsic electrostatic discharge protection.
However, detonators using such materials have proved to be unreliable. Unlike CP, these powders frequently decouple from the bridgewire, resulting in ignition failure. Further, detonators that contain HMX, RDX, PETN, or other secondary explosives are prone to ignition and growth-to-detonation failures because powder confinement is a critical and sensitive parameter.
Studies have shown that mechanical confinement of the powder is necessary to prevent the decoupling that occurs with increasing time or thermal cycling. Elimination of the decoupling by mechanical means has not been proven to date. In addition, growth-to-detonation in such devices is sensitive to physical characteristics of the powder (particle size, surface area) and occurs more gradually than in CP detonators. As a result, reliability of growth-to-detonation is diminished.
Consequently, a need exists for a fresh approach to providing a spark-safe and bonfire-safe, low-voltage detonator that will avoid the above-described problems associated with previous attempts.